1. Field of the Invention
The present invention relates to a lens, more particularly to a light guide lens and a bicycle headlight including the light guide lens.
2. Description of the Related Art
Conventional light guide lenses applied in bicycle headlights (for example, that discussed in U.S. Pat. No. 5,757,557) mostly adopt a symmetric light output design, in which the light guide lens includes an outer surrounding surface that is symmetrical with respect to an optical axis (Z). Thus, the light output of the light guide lens has a symmetric distribution relative to a reference plane transverse to the optical axis (Z). However, in consideration of light output efficiency and driving safety of oncoming vehicles, etc., many countries have issued traffic regulations in which the light output of bicycle headlights should comply with a specific distribution satisfying traffic safety. For example, referring to FIG. 1, a test board 1 for testing the light output distribution is adopted by the German traffic affair authority. The German traffic affair authority requires that light source luminance flux of a to-be-tested vehicle headlight should not exceed 42 lm (lumen). The test board 1 is at a distance of 10 meters from the to-be-tested vehicle headlight, and includes seven test points and one test zone. The test board 1 is explained in the following:
1. the test board 1 including a HV test point 101 which is an intersection of a horizontal line (H-H) and a vertical line (V-V), an imaginary line that interconnects the HV test point 101 and a center of the vehicle headlight being transverse to the test board 1, other test points being determined according to included angles between the imaginary line and a connecting line that interconnects the vehicle headlight and one of the test points on the horizontal line (H-H) or the vertical line (V-V);
2. if an illumination value tested at the HV test point 101 is a maximum value (i.e., a maximum illumination), a value of the maximum illumination being at least 10 lx (lux); if the maximum illumination is not tested at the HV test point 101, the value of the maximum illumination should not exceed 1.2 times the illumination value at the HV test point 101,
3. a L1 test point 102 being determined according to an included angle at 4° on a left side with respect to the HV test point 101, a R1 test point 103 being determined according to an included angle at 4° on a right side with respect to the HV test point 101, and a second test point 104 being determined according to an included angle at 1.5° below the HV test point 101, each of illumination values tested at a respective one of the three test points 102, 103, 104 being required to achieve 50% of the maximum illumination,
4. a third test point 105 being determined according to an included angle at 5° below the HV test point 101, an illumination value tested in an area between the second test point 104 and the third test point 105 being required to exceed 1.5 lx;
5. a L4 test point 106 being determined according to an included angle at 4° on a left side with respect to the third test point 105, a R4 test point 107 being determined according to an included angle at 4° on a right side with respect to the third test point 105, each of illumination values tested at a respective one of the two test points 106, 107 being required to exceed 1.0 lx; and
6. the test zone 108 being determined according to an included angle greater than 3.4° above the HV test point 101, an illumination value tested therein should not exceed 2.0 lx.
It is known from the aforementioned requirements of the German traffic affair authority that the light output of the to-be-tested vehicle headlight should have an asymmetric distribution exhibiting a brightest part in center, and gradually darker parts above and below the center relative to the test board 1. Thus, the conventional headlights with symmetric light output distribution are not capable of satisfying current requirements. Even though light reflective components may be adopted to adjust the light output distribution, light output efficiency would be adversely affected.